Abstract : This study describes an integrated analytical framework that captures the major economic, engineering, geographic, and social factors affecting the internal (direct) cost of abyssal seafloor waste isolation. We develop and apply computer models based on this framework to produce cost estimates for the disposal of sewage sludge and municipal incinerator ash via four deep ocean waste emplacement system concepts (surface emplacement, ROV glider, direct descent disc, and pipe riser) developed by Oceaneering Technologies. Our study focuses on five metropolitan areas (New York, Miami, Galveston, Los Angeles, and Seattle) and five proposed abyssal study sites identified by the Naval Research Laboratory's Abyssal Plains Waste Isolation Project, of which this work is one component. Evidence suggests that the external (human and environmental health) costs of abyssal waste isolation are small compared to internal costs. However, the variance of external cost is large because our present state of knowledge about the fate and effect of waste in the ocean does not permit us to predict outcomes with certainty. Reducing this uncertainty is an important objective for future research. Assuming that external costs are not significantly higher, and that the marine transport concepts do not prove to be significantly more expensive than anticipated, abyssal ocean waste isolation would cost about $43 per ton for sludge and ash from the New York metropolitan area. This is competitive with present land-based disposal costs in New York City of over $160 per ton for sludge and over $48 per ton for ash. The abyssal ocean option may be less competitive in other metropolitan areas because of their more limited waste volumes. (AN)